1. Field of the Invention
The present invention relates generally to the field of data communications.
2. Related Art
With the advent of the personal computer and the tremendous popularity of the Internet and on-line services, the number of computers connected to the public switched telephone network (PSTN) has grown immensely over the past decade. It is estimated that 20-30% of all calls placed on the telephone network are established for the purpose of allowing one terminal or computer device to communicate with another computer device. These calls are known as data calls. The characteristics of a data call are unlike the characteristics of voice calls. A voice call normally lasts for 3 centennial call seconds (CCS), which is about five minutes, whereas a data call normally lasts for 36 CCS (about an hour). This presents a problem because the telephone network was not designed for handling the relatively long duration data calls. Consequently, as result of the tremendous increase in the number of data calls served by the phone network, the network is increasingly being overloaded.
FIG. 1 illustrates a representative overloaded PSTN 102. PSTN 102 comprises a plurality of central office switches (CO) 110, 112, 114, 116 and at least one STP/SCP node 118. Each CO has a serving area, which is the geographical area in which the CO is located: all subscribers in that area are served by that CO.
FIG. 1 shows a user 103 that desires to connect data terminal device 104 with remote data terminal device 124 using PSTN 102. Data terminal device 104 is connected to PSTN CO 110 through data communication device 106, such as a modem, and dial media 108.
In order to establish a connection between data terminal device 104 and remote data terminal device 124, data terminal device 104 directs data communication device 106 to place a call to remote access server (RAS) 120 using PSTN 102. Data communication device 106 places a call to RAS 120 by sending a call request to PSTN CO 110. Upon receiving the call request from data communication device 106, the PSTN establishes a circuit from the originating CO 110 to RAS 120 through terminating CO 114. RAS 120 is connected to data network 122, which is connected to remote data terminal device 124.
RAS 120 provides full data call establishment by performing the reverse of the processes performed by data communication device 106. The processes performed by data communication device 106 includes the processes of: (1) data compression; (2) error correction; (3) link layer framing; and (4) modulation, in that order. Thus, RAS 120 provides full data call establishment by performing the following steps in the following order: (1) demodulation; (2) link layer framing; (3) error correction; and (4) data decompression.
Modulation refers to the conversion of a binary bit stream into a modulated signal within the voice frequency range. The facilities of a PSTN are designed to handle voice traffic, not binary data. Thus, to transmit binary data through the phone network it is necessary to perform the process of modulation. The modulated signal is then used to xe2x80x9ccarryxe2x80x9d the binary data through the phone network. Demodulation refers to the process of converting a modulated signal back into the original binary data. Consequently, a modulator/demodulator (i.e., modem) is necessary to transmit binary data from one computer to a second computer through the phone network.
The process of link layer framing refers to a process of encapsulating data within a frame for transmission on the physical layer. Encapsulating data within a frame enables the error correction processing.
After the call is established by RAS 120, data communication device 106 accepts user data from data terminal device 104 for transmission to RAS 120. Data communication device 106 prepares the user data for transmission over the PSTN by first encapsulating the data in a protocol (such as PPP), compressing the encapsulated data, applying error control, framing the data in a link layer frame, and modulating the link layer frame. RAS 120 receives the modulated signal, demodulates the signal to recover the link layer frame, removes the link layer framing, checks for errors, decompresses the data, and de-encapsulates the call to recover the user data in its original form. The user data is then forwarded to remote data terminal device 124 through data network 122.
The circuit set up between CO 110 and CO 114 remains in use until data communication device 106 terminates the call and releases the circuit, regardless of whether actual data is being transmitted. Thus, valuable PSTN circuits are consumed from data communication device 106 to local CO 110, between originating CO 110 to terminating CO 114, and from terminating CO 114 to the RAS.
To conserve valuable PSTN circuits, what is needed is a system to bypass the PSTN by capturing data calls at the originating CO and transmitting the compressed user data associated with the data call through a data network to a device that will then decompress the data and transmit the decompressed data to the intended destination.
In a system wherein a data communication device receives user data from a data terminal device, compresses the user data, encapsulates the compressed user data within a link layer frame, and transmits a modulated signal corresponding to the link layer frame to a switch within a telephone circuit switch network, the present invention provides a system for transporting the compressed form of the user data through a data network, thereby bypassing the telephone network.
The present invention includes a remote access concentrator (RAC) connected to a network access controller (NAC) through the data network. The RAC is connected to the switch within the telephone network and includes a network interface for receiving the modulated signal from the switch. The RAC also includes a demodulator to demodulate the modulated signal so as to recover the link layer frame. After recovering the link layer frame, the RAC tunnels the link layer frame through the data network to the NAC. Since the link layer frame contains the compressed form of the user data, the compressed user data is transported through the data network.
The NAC receives the tunneled link layer frame from the RAC and extracts the compressed user data from the link layer frame. The NAC then decompresses the compressed user data to recover the user data in its original form. The user data is then processed by the NAC according to the user data type. Finally, the NAC forwards the user data to the remote data terminal device.
The invention supports a variety of user data types, including: Asynchronous data, Point to Point Protocol (PPP), and Serial Line Internet Protocol (SLIP). The invention""s ability to support a variety of data types is based on the RAC tunneling the link layer frame to the NAC, such that the RAC does not directly process the user data.
In a first embodiment of the present invention, the switch within the telephone network is a CO. In a second embodiment of the present invention, the switch is a Competitive Local Exchange Carrier (CLEC) switch.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.